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Americas Peopled by One Tribe?

24 May 2005 (All day)
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Descendants. Distant, ancestral cousins of this Chinese family may have helped people the Americas.

The number of humans that first colonized the Americas may have been as few as those in a single tribe, around 200 or 250 people, a new population genetics model study suggests. This figure, somewhat lower than past estimates, is so small that some researchers think it unlikely since such a group would have been likely to perish.

Migrations into the Americas are a hotly debated topic, with archeologists, linguists, and geneticists arguing over how many migrations there were, where the immigrants came from, and when they arrived. Most researchers agree that the migrants came in at least two waves. The first arrived sometime before 14,000 years ago, then multiplied and fanned out across North and South America.

Population geneticist Jody Hey of Rutgers University in Piscataway, New Jersey, was curious about the size of this founding population. He created a mathematical model that traced an ancestral Asian population splitting in two, with the splinter group venturing across the Bering Strait. Hey plugged DNA sequences from modern East Asians and Native Americans into his model. He focused on DNA regions that showed no sign of recombination or certain types of natural selection; such regions can reveal how populations diverge genetically after they split. By examining how differences between the East Asian and Native American DNA and the variations within each group, Hey estimated how many people were among the first wave of migrants.

The varied DNA sequences all suggested that the founding group had about 70 breeding individuals, Hey reports online this week in PLoS Biology. This is fewer than in past estimates based on demographic reconstructions and simpler genetic analyses, for example those based on blood group markers, which suggested the founding group had about 100-1000 breeding individuals. Using a rule of thumb to account for the number of children and others not breeding, the new estimate translates to a group of only about 210 to 250 people.

This is a "solid but initial effort" with an intriguing result, says molecular anthropologist Theodore Schurr of University of Pennsylvania in Philadelphia. "This is not a lot of folks" to begin populating a continent, he says. Rebecca Cann, population geneticist at the University of Hawaii in Manoa, says the estimate is unlikely because such a small group would be in danger of dying off en route. She is critical of the model, adding that migration to Hawaii from the South Pacific about 2000 years ago is easier to reconstruct, and yet "it is hard to get to get anything more concrete than 100 to 10,000"for the founding population's size.

Related sites
Hey's PLOS study
Hey lab Web site
Theodore Schurr's home page
Rebecca Cann's home page

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